Nitric oxide (NO) is a gas that, when inhaled, acts to dilate blood vessels in the lungs, improving oxygenation of the blood and reducing pulmonary hypertension. Because of this, nitric oxide is provided in inspiratory breathing gases for patients with pulmonary hypertension.
Often, apparatuses used for nitric oxide drug delivery provide a direct ppm dose setting based on patient inspired breathing gas in order to provide a constant concentration within the breath period. As the flow rate of breathing gas rapidly rises and falls within the inspiratory or expiratory phases, it becomes difficult to provide a proportional ratio-metric dose of delivered NO gas dependent on inspired flow.
Methods of closed loop proportional flow control which are utilized to titrate the desired dose have limitations regarding dynamic range and impulse step response to changes in breathing gas flow. The primary limitation is known to be at the extremes of the NO flow control range, i.e. lower than 1% or greater than 100% of the NO flow control range. Typical flow control technologies including electromagnetic valve in combination with flow sensor and microprocessor running PID (proportional, integral, derivative) control are utilized for wild stream blending of gases. On the lower 1% end of the control range, integral error is minimal to drive sufficient control system proportional valve gain. An over-damped system, combined with poor flow control valve step response can under-deliver the desired NO gas for a significant portion of the breath period. At the opposite extreme, when peak inspired flows exceed 100% of the NO flow control range for a significant portion of the breath period, there is also under-delivery of the set dose. Additionally, a highly tuned, proportional control, fast response control system combined with a large hysteresis, or a poorly acting proportional control valve can act to over-deliver the desired dose when operating in the lower 1% of the control range.
In fact, some delivery apparatuses shut down automatically when the calculated ratio-metric amount of NO flow from the inspired gas flow is found to be greater than 2 times or less than one half of the desired ppm set dose. When supply of nitric oxide is abruptly cut off, patients may experience adverse effects such as worsening of partial pressure of oxygen in arterial blood (PaO2) and increasing pulmonary artery pressure (PAP).
Variability or irregularity in an unknown inspired flow profile from a support device such as a breathing gas delivery system may produce such flow conditions, and when combined with insufficient dynamic proportional control range, may then result in shutdown of the inhaled NO delivery system or other NO delivery apparatus. Additionally, current inhaled NO delivery systems have insufficient dynamic delivery range and cannot be used with gentle ventilation as gentle ventilation often requires lower flows than conventional ventilation. This can, again, result in shutdown of the delivery apparatus with resulting rebound hypertension and oxygen desaturation, which may result in adverse events as serious as death.
In addition, nitric oxide delivery system architecture provides complete independence of NO gas delivery from gas concentration measurement within the inspired limb of the patient circuit. Traditionally, gas concentration measurements are displayed in ppm on the main screen of the device with NO proportional delivery control performance suppressed. With this independence, when NO set dose is not equal to the reported concentration measurement, the user has difficulty in assessing which portion of the system monitoring or delivery is performing poorly.
Therefore, there is a need to monitor and display the flow(s) from the patient support device to provide safe delivery of nitric oxide, as well as a need to provide the user a method of determining limitations of dynamic ratio-metric gas NO delivery blending performance.